<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>left/right拉伸和width同时存在</title>
<style>
.image {
    position: absolute; left: 0; right: 0; width: 50%; 
}
.button {
	padding-top: 200px;
}
.button input {
	width: 280px; height: 40px;
	font-size: 20px;
}
</style>
</head>

<body>
<img class="image" src="http://img.mukewang.com/547c34c9000171a002560191.jpg" height="191">
<p class="button">
	<input type="button" value="添加margin: auto;" onClick="document.getElementsByTagName('img')[0].style.margin = 'auto';">
</p>
</body>
</html>